This invention relates to the conversion of organic solid waste materials, such as wood, rubber, sewer sludge, agricultural residues, refuse derived fuel and hazardous waste, into a more usable form including medium BTU gas, high energy chars and condensible oils. More particularly, this invention relates to the pyrolysis and gasification of a feed material in an indirect fired self-sustaining rotary kiln reactor provided with an internal recycle system to enhance the conversion processes.
Several different methods are available for the thermochemical treatment of waste products, including various combustion gasification and pyrolysis processes. One such gasification process utilizes an indirect fired rotary kiln in which carbonaceous waste materials are gasified by pyrolysis and classical carbon, hydrogen and oxygen chemistry. Pyrolysis is the high temperature chemical decomposition of the waste material in an oxygen free environment producing solid, liquid and gaseous products. The carbon, hydrogen and oxygen work to modify the conversion fractions of the resulting components. The principle reactions are the steam char reactions, brodeaux reactions, and the water gas shift reactions. These reactions work to convert the resulting char to gas, and to produce higher hydrogen concentrations.
The alkalai elements in the ash catalyze these reactions, driving the products toward more gas and less solids. Increases in the residence time will also aid in increased gas production and reduced char values.
The pyrolysis process results by providing a high temperature reaction chamber where each feed particle is heated rapidly to destruction. The chamber is the rotary kiln and the heat source is radiant tubes mounted in the reactor. The radiant tubes are fired with part of the gas produced which is supplied by bringing cleaned product gas on board the reactor. The feed material is supplied to one end of the reactor, and the forces of gravity provide the material flow of the solids to the outlet end.
The system design provides for good energy utilization since the flow of solids is counter to the flow of gas in the radiant tubes. However, the percentage of gas used to heat the reactions and the residual char levels can be high. To improve these operating characteristics, the present invention adds several features to the rotary kiln design. An internal recycle system is provided to return part of the char produced to the fuel end of the reactor. The recycling of the char (carbon and ash) increases the char residence time in the reactor permitting increased time for conversion to gas. At the same time the char fraction in the reactor is increased, increasing the carbon and ash concentration at any point in the reactor. The increased catalytic ash content together with the increased carbon concentration drive the reactions' products toward more gas and less solids, while the increased carbon concentrations also drive the reaction products toward gas.
These conversions are further aided by the increased temperature at the cold end resulting from mixing the hot, recycled char with the incoming feed. These higher temperatures also result in a better conversion to gas, while reducing the amount of gas required for the radiant tubes to thereby improve efficiency.
Another feature which aids in gas production is the reduction of the condensible gas fraction in the product gas. This is accomplished by recycling part of the produced gas to increase the gas residence time, and at the same time transferring a portion of the sensible energy of the hot gas to the raw feed to increase the heating rate in the reactor. The gas recycle means extends through the center recycle tube, aiding the solid recycle by helping to increase the velocity component of the solid particles and permitting further reactions within the recycle housing.
In summary, the gas and char recycle system aids in reducing these components while increasing the product gas fraction. The recycle system also conserves valuable energy by reducing the product gas requirements for the burners and reduces the heating length in the reactor favoring faster and more complete gasification.